Practical tips for adding LED rate indication to trem circuit?

[King Fan]

I like my trem amps (5G9, 6G2, PR, VC) but it'd be handy to have a visual rate indicator. Merlin shows us how -- *on the schematic*. He says: "By replacing the cathode resistor with an LED we eliminate the need for the large bypass capacitor, and obtain maximum gain and minimum output impedance at all frequencies in one fell swoop. A red LED will be suitable in most cases... the LED will flash in time with the trem' signal... the circuit (drawn) shows this, using an ECC81 this time."

Here's the base drawing:



and here's the LED version:



I'm guessing the ECC81 isn't a necessary part of this mod. But I'm left wondering how to do this in an actual amp, in layout space.

Take the 6G2 for example.

1. Lemme confirm: the LED goes on V2 pin 3, yes?
2. Would I have to drill a hole in the faceplate for the LED? *Or* I've heard you can position the LED to shine in next to the pilot bulb so the pilot light will pulsate with the LED. But that seems like a long wiring route -- and like it'd be darn tricky to orient the light(s).

Any practical experience or advice would be welcome.

 

[Wally]

I’ll bump.
Audio function…doesn’t need a light????? I would not drill a hole in the front panel of any of those amps. Ymmv.
That said and knowing that I may be entirely incorrect here…
drill the hole. Install the LED in between the Speed and the Intensity. Run the wiring back to those two points on the board. Make it neat….but I don’t think there will be a problem…until someone asks why you put holes in the front panels of those amps. (;^)
How about this…build a light panel that sits above the stage. These lights will respond both to pitch of the notes played…different colors for different pitches….as well as pulsate to the trem. You can take the signal out of a hole in the back panel. Now you have a light show worthy of watching….think the Rickenbacker Light show guitar.…..
Nah…it would be easier to use liquid on an overhead projector to do a ‘San Francisco’ light show like back in the day…..something that could capture the imagination and expand the consciousnes.

 

[muscmp]

i think it's a cool idea. i'd even like a magic eye tube-like led that acts that way.

play music!

 

[corliss1]

@muscmp - this is all your fault - I just read about magic eye tubes for a half hour I kind of want to do a trem project with one now.................

 

[King Fan]

Audio function…doesn’t need a light????? I would not drill a hole in the front panel of any of those amps. Ymmv.
That said and knowing that I may be entirely incorrect here…
drill the hole. Install the LED in between the Speed and the Intensity. Run the wiring back to those two points on the board. Make it neat….but I don’t think there will be a problem…until someone asks why you put holes in the front panels of those amps. (;^)
How about this…build a light panel that sits above the stage. These lights will respond both to pitch of the notes played…different colors for different pitches….as well as pulsate to the trem. You can take the signal out of a hole in the back panel. Now you have a light show worthy of watching….think the Rickenbacker Light show guitar.…..
Nah…it would be easier to use liquid on an overhead projector to do a ‘San Francisco’ light show like back in the day…..something that could capture the imagination and expand the consciousnes.

Audio function, Wally, yes, but the LED is there just to indicate the trem speed so I can set it by eye, instead of having to listen to the trem to adjust. Maybe I left out the key sentence in the Merlin piece: “Additionally, because the oscillations extend to cut-off, the LED will flash in time with the trem signal giving a novel and useful visual indication of the frequency.”

And I agree it’d be a pain to drill a tiny hole in the control panel (these aren’t vintage amps) — but heh, the light doesn’t do me much good if I can't see it. That's why I liked the idea I read about once of orienting it to shine into the gap behind the pilot light lens, said to make the combined light pulse in time with the trem. But I’ve never seen a pic of the placement or the wiring, or heard any practical advice. That’s mostly what I was hoping to learn.

Of course now that the Magic Eye has made an appearance, my little LED quest is probably doomed. I first saw one of those on a big reel-to-reel deck a friend had in high school. Fascinating.

As for the liquid light show on the overhead projector, Wally, we did that in high school too. Mostly expanded our consciousness of how rotten the colorful oils smelled as they got hot and evaporated… more nauseating than fascinating. But let me stress I’m interested in the “useful” not “novel” or mind-expanding properties of this LED.

 

[tubeswell]

View attachment 1077267

I'm guessing the ECC81 isn't a necessary part of this mod.

No but you need a minimum gain of about 30 (or preferably more) for it to oscillate, so don't waste time with something weaker than a 12AT7 triode. Drill a hole in the face plate of the chassis for the LED grommet and hook up two wires. Keep them away from high VAC leads. Thats about it

 

[24 track]

there is an opto isolated flashing Neon light and a photo resistor for the tremolo circuit in fender amps ( there may be a way to utilize that circuit to drive a low powered LED as an indicator ), it depends on how much current/ voltage to the neon bulb and would this mod create havok with that circuit, dependant on the voltage, a limiting resistor would have to be calculated to drive the LED if this idea works . never tried and I hate mods , they never work right plus most can never be undone.

 

[Wally]

I have one or more magic eye tubes somewhere in all of these boxes.

 

[lowatter]

Any possible way to have a flashing LED wired/mounted somehow to the footswitch?

 

[Paul-T]

I have a flashing trem speed light on my surfyverb - it's a really helpful visual clue to the setting. You can see modern LEDs with that little glass lens a pretty long way away.

 

[King Fan]

No but you need a minimum gain of about 30 (or preferably more) for it to oscillate, so don't waste time with something weaker than a 12AT7 triode. Drill a hole in the face plate of the chassis for the LED grommet and hook up two wires. Keep them away from high VAC leads. Thats about it

Thank you, sir, now we’re getting somewhere. I actually ordered some LEDs and grommets a few years back, last time I got this urge. I’m gonna go research LED wiring (polarity?) but if someone has an easy answer (like “there isn’t any” or “cut the red wire; no wait, the blue wire”) it'd make this thread more complete…

EDIT: See next post…

 

[King Fan]

Hold on. Google works: “LEDs, being diodes, will only allow current to flow in one direction. And when there's no current-flow, there's no light. Luckily, this also means that you can't break an LED by plugging it in backwards. Rather, it just won't work…. The positive side of the LED is called the anode and is marked by having a longer lead, or leg. The other, negative side of the LED is called the cathode. Current flows from the anode to the cathode and never the opposite direction.“ Duh. I suspect everyone younger than me learned that in 5th grade. We were still working on making fire with sticks…

 

[King Fan]

Got DIYLCOCD? Just did a little schemogram for the 6G2, which may become the test bed... Yes/no?

 

[King Fan]

Oh, and for the other 2.5 people here who don't speak LED, I now recall I learned this a few years ago and forgot it...

"Try to find the longer leg, which should indicate the positive, anode pin. Or, if someone's trimmed the legs, try finding the flat edge on the LED's outer casing. The pin nearest the flat edge will be the negative, cathode pin."

 

[King Fan]

And just in case the horse isn't completely dead, here's the type of LED bezel I ordered from Doug Hoffman -- he helpfully notes it fits in a 5/16" (~8mm) hole.

 

[doof]

"Try to find the longer leg, which should indicate the positive, anode pin. Or, if someone's trimmed the legs, try finding the flat edge on the LED's outer casing. The pin nearest the flat edge will be the negative, cathode pin."

Some years ago I got a baggie of cheap LEDs with no flat side and even-length leads. Eventually I was able to bake it into my brain that the larger, flag-shaped terminal inside the LED casing was the cathode. So for me, the flag always marks the cathode.

 

[WalthamMoosical]

In fact, sometimes the leads are deliberately manufactured with the cathode lead longer. You can buy them by the hundreds this way. (Ask me how I know.) Look for the flat side--it's a much safer way of determining where the cathode lead is. No flat side? See previous post, I guess.

 

[King Fan]

Excellent, gentlemen, thanks. Those are valuable tips. And I guess the final polarity test is *does it work*?

Now, it turns out my dead horse wants to be fed. Hmmph. The nice little LED bezels Doug Hoffman sells come with two tiny bits of plastic. The larger has two tiny holes with tiny '+' and '–' labels; it fits perfectly in the sleeve of the bezel. I'm thinking it may be to insulate the leads as they go through the bezel

But then there's a much tinier one with (of course) a single hole. Hmm; maybe the tiny one adds protection to the anode / + lead? Here's my guess, assuming the long leg is (+). But I have no real idea, so if you know, throw the horse a handful of oats.

 

[King Fan]

And yep, up close, if we 'follow the flag' as per @doof , then the short lead here is the cathode. No flat side, I think.

Nice horse, good horse. Here's a carrot and a big plug of chaw. (Strange but true: Horses are crazy about chewing tabaccer).

EDIT: Oh wait, there is a flat side. How do you spell "subtle"?

 

[corliss1]

I haven't ever used the shorter one you show, and not sure I've ever seen one.

I usually put a dab of silicone or something to hold it all together, since I have found those holders don't always lock in very well.

I find these to be much easier to deal with for panel mounting - drill the right size hole, and snap it in: https://tubedepot.com/products/red-snap-in-panel-mount-led